In drilling wells for oil and gas exploration, understanding the structure and properties of the geological formation surrounding a borehole provides information to aid such exploration. However, the environment in which the drilling tools operate is at significant distances below the surface and measurements to manage operation of such equipment are made at these locations. Further, the usefulness of such measurements may be related to the precision or quality of the information derived from such measurements.
Remote sensing and evaluation of subterranean reservoirs has been important in oil exploration and production, since it provides important information that would otherwise be available by expensive drilling operations. The widest-scale remote sensing application is the seismic survey that is conducted from the surface to locate oil reservoirs. Although these surveys are important in developing drilling strategies, they only provide very coarse information that needs to be complemented by other shallower and more detailed evaluation tools that are located down-hole. As an example, downhole sensors placed in a vertical well can be included in addition to the sensors at the surface to achieve more effective sensing.
Many existing reservoir imaging and monitoring schemes operate in two configurations. The first configuration is a cross-well imaging configuration, in which the sensors are placed in multiple vertical wells and data is collected from the volume that is surrounded by these wells. The imaging is performed in between the vertical wells drilled at different locations surrounding the volume of interest. Developed first for conductivity mapping, these sensors have been extended to resistivity mapping. The second configuration is a surface configuration, where the transmitters and receivers are located at the surface or at very shallow depths. Although these configurations have been successfully used in imaging shallow reservoirs or reservoirs with simple geometry, they have exhibited difficulties in deeper and more complicated conditions. Further, since reservoirs that have been relatively easier to access have been drilled out in the past years, the industry is faced with reservoirs that are harder to explore and produce.